Unique Email Sync Issue with Outlook and Android

We recently switched hosting providers and ran into an issue their advanced tech support team had never came across.

Originally, we had a AlmaLinux 8.x installation on a VM at Google Cloud. Between the high costs, advanced knowledge requirements that I lacked in Google Cloud, and personal stress performing your own updates to a Linux repo, it was a clear sign that I was not meant to be a Systems Administrator. We migrated to InMotion Hosting were we decided to upgrade (for half of the cost of running it myself) to a dedicated server instead of a VPS with the same OS. There migration team handled the migration of all of our accounts, once done, they notified us to update the DNS of the primary domain. Upon update, the process actually moved rather quickly.

Forward to the start of the next day, I am getting calls that their emails are missing from the period during the migration. I knew that they would go back in and re-sync those from the old server, no problem. One customer stated they were having an issue where MS Outlook (which was using the original configuration from before the migration) and a Android phone. When an email would come into the server, we could see it on RoundCube webmail. Outlook 365 would not automatically sync. I thought it was weird, but what really was weird (and alarming) was that when the email client on the Android was refreshed (swipe down from the top), the new email would disappear and that would sync to the server.

Most of the support agents brushed it off as they assumed I was using POP3, but indeed I confirmed it was a IMAP connection. The agent stated they wouldn't be able to help as it sounded like a 3rd party software issue (and I assumed they were right at first). Before we disconnected from support, I noticed the cPanel account's Email Deliverability status indicated there was a problem for all domains. I asked the agent and they logged in as root to the WHM and confirmed the same and noticed our Helo, PTR, and Hostname were incorrect. After correcting these, Outlook suddenly began it's usual sync (every 30 seconds) and those previously deleted emails reappeared (thank the email gods).

The agent said she had never seen anything like this before in her life. I don't imagine the client's sync schedule would have had anything to do with this but I'm also not a SysAdmin. Does anyone have any idea why this happened and why it would have caused mail clients to not properly sync or have emails disappear and reappear?